System for creating, reading and writing on rotatable information storage media, an apparatus for performing both read and write operations

ABSTRACT

An apparatus for reading and writing information on a laser writable rotatable storage medium employs a single laser source. Part of the laser beam is diverted and made to move in an oscillatory manner across an edge of an embedded track. The other portion of the beam is used for reading and writing. To avoid overloading reading detector circuits, an enable switch is provided which responds to read/write control signals.

BACKGROUND OF THE INVENTION

The present invention is generally directed to the manufacture andutilization of information storing systems employing rotatable storagedevices which contain a reference track which renders it possible toperform multiple storage and retrieval functions including theidentification of individual bit locations on a recordable disk. Moreparticularly, the present invention is directed to an apparatus for bothreading and writing operations using a single laser source. Even moreparticularly, the present invention is directed to a mechanism whichprevents overloading detector circuits during necessarily higher powerlaser writing operations.

A central aspect of the present invention involves the recognition of afrequency doubling phenomenon that occurs when a light source having anessentially gaussian intensity distribution is dithered across a surfaceexhibiting a sharp reflectivity discontinuity (or other optical propertydiscontinuity). The present invention exploits frequency doubling andphase reversal as mechanisms for accurately positioning a laser lightsource which is used to write information onto a recordable, rotatableblank in systems which can be made to be compatible with existing CD-ROMdevices which unfortunately only perform read functions. While there doexist apparatus for performing disk-writing operations, these systemsare expensive and cannot provide the information density which isrendered possible by the use of a servo tracking system of the kinddescribed herein.

Clearly, the commercial craving for increased information density onrecordable media has been a significant driving force in present dayinformation-handling systems technology development. Moreover,commercial and market forces have increased the demand for a significantincrease in the raw amount of data needed to be stored and accessed upona removable device. These forces have also increased the demand thatthis information be available quickly and accurately. Moreover, therehas been an increase in the desire to make the information on thesedisks writable by end users.

Other related developments in this technology have included the designand construction of units that are capable of writing information toseveral overlying layers in a single compact rotatable disk device.Furthermore, the desire to store graphic and moving picture images andsound on these devices has also produced variants of the now ubiquitouscomputer and audio CD-ROM devices and the market has now branched outinto the production of DVD (digital video disk) platters. Accordingly,it is an object of the present invention to provide solutions toproblems which occur in this emerging technology.

The present invention provides and is an overall solution to theproblems and challenges produced by digital information storagetechnologies. The present invention provides a comprehensive anddetailed solution to many of the problems existing in this industry. Itis seen in the present invention that the inventors have started from atechnology base related to servo positioning and have extended thatcapability and have utilized it to provide an extensive andcomprehensive solution to all of the above-described problems of datastorage, transmission speed and data accuracy. A complete end-to-endsolution of the problems described is provided herein. It is thereforeseen that the present inventive system begins with the production ofinformation storage blanks which are employed in applicants' inventivesystem. Associated with these blanks, there is also included anapparatus for reading and writing to these disks. An additionalbeneficial mechanism that falls out from this work is a system forimmediately performing read-after-write operations. This is possiblesince the signals produced by the servo tracking mechanism of thepresent invention are particularly useful for exceedingly accurateposition control. Additionally, deliberate dithering and its signalconsequences are employed to control focus for reading and writingmulti-layer disks. Further improvements also result from the fact thatlaser beam splitting is employed to write information to both sides of adisk using simple and relatively inexpensive beam splitting devices. Inall of these variations, applicants have been able to exploit thesignals that are produced using specifically designed detector circuitsfor performing a variety of different functions. In particular, thenotions and problems associated with error correction take on anentirely new meaning and perspective when performed within the contextof the system claimed herein.

The detailed description below includes a complete characterization ofall aspects of making and using the specific system or method claimedherein. Various aspects of this description can, however, be found asbeing described under the following general topics; accordingly, thedetailed description is divided into several sections in which thefollowing specific aspects are discussed: (I) Recording Blanks; (II)Basic Writing Operations; (III) Signal Detection; (IV) Reading afterWriting; (V) Error Handling; (VI) Multi-layer Processing; and (VII)Read/Write Head Design.

To fully understand the advantages provided by the present invention,one should appreciate the problems presently found in CD-ROM writingtechnology. A significant problem in this area is the fact that thedisks for writing are not inexpensive. Furthermore, a fundamentalproblem in writing to CD-ROM-like devices using a laser is that there isa definite need for data formatting. In present day CD-ROM writingtechnologies, all of the data medium looks the same. There are noreference points per se. Efforts at resolving this problem by providingmolded grooves in a disk as a starting point are disadvantageous in thatthey are large and consume a significant amount of space on the disk.Other approaches which attempt to solve this problem employ masterdisks. These disks, however, require expensive mastering systems and thetracks are still too large or at least larger than needed. Therefore, itis seen that a significant need in the CD-ROM writing technology is amechanism for providing inexpensive writable media.

Other problems in this area are also presented. In particular, it isknown that there are, in fact, different data formats employed. Forexample, one data format may be employed for an audio CD-ROM, anotherformat may be employed for a CD-ROM which stores computer program anddata. Yet another data format may be employed for the storage ofreal-time video presentations such as is seen in the recently announcedDVD technology (DVD stands variously either for Digital Video Disk orDigital Video Data).

Furthermore, in the midst of the technology explosion related to CD-ROMtype devices, various implementations of multi-layer disks are beingproposed.

Yet another problem with any advanced CD-ROM writing technology is thequestion of whether or not it is compatible with already existing CD-ROMreading technology. Furthermore, still further questions arise withrespect to whether or not future technologies such as is provided by thepresent invention are compatible with systems such as magneto optical(MO) recording techniques. Fortunately, the system provided by thepresent invention is compatible with many existing technologies andprovides break through approach in which ultimately each bit on a CD-ROMis addressable and locatable. As a consequence, the problems associatedwith disk formatting disappear. Effectively, such problems are relegatedto software, leaving the hardware free to easily express information ina variety of formats.

SUMMARY OF THE INVENTION

The fundamental principle behind the present invention is theutilization of optical dithering employed in conjunction with theprovision of sharply different optical disk regions. These opticallydifferent regions form the basis of providing a trackable servo spiral(or concentric) track whose edges define paths for control of laserreading and writing. Furthermore, in situations in which multi-layereddisks are employed, dithering across an edge of the track issupplemented with dithering in the simultaneously provides the abilityto select the layer of the disk into which information is written andfrom which information is read. In this regard, it is particularly notedthat, not only is this layer selection rendered possible, but controlledfocus into the desired layer is also provided via the same opticaldithering principles used for servo tracking in the radial orre-direction.

The optical dithering principles employed in the present invention aresignificant because they result in a method for tracking both forwriting and reading purposes in which it is possible to determine notonly that one is "on track," but also that one can determine exactly howfar off one is with respect to an edge of an embedded servo track. Thisis to be contrasted with other systems in which only off-track errorindications are provided. The present system provides continuouspositive feedback control which precisely determines and controls thelocation of a laser spot either for writing or for reading purposes. Itis this precision which permits multiple information tracks to be readfrom and written into the recordable medium which lies between embeddedservo tracks.

The positional precision plus the phase of the output associated withdithering on one side or another of a servo track side permits trackcounting and, therefore, provides an accurate determination of theradial position of any point on the recordable medium. This fact coupledand used in conjunction with one or more registration marks preciselydetermine the position of a bit in the angular or θ direction, θ being apolar coordinate position angle.

A number of advantages flow from this capability including thecapability of providing the system with drive system diagnostics andmonitoring together with the monitoring of the performance of the laserlight source employed. Furthermore, the precision that results from theuse of the present invention permits errors to be corrected in ways thatwere not heretofore possible with other systems. This is particularlytrue when the principles of the present invention are employed inconstructing read-after-write configurations. And again, theseconfigurations are rendered possible because of the specific nature ofthe tracking mechanisms provided herein. It is furthermore noted thatthe ability to know precisely where information is on a recordableCD-ROM-like medium means that the disk may be indexed much like a randomaccess or magnetic disk memory. This means that information on disksused and manufactured in accordance with the present invention may befound rapidly and stored densely and efficiently.

Accordingly, it is an object of the present invention to provide acomprehensive system for writing information on inexpensive rotatablemedia similar to current CD-ROM technology.

It is also an object of the present invention to provide a method forproducing blank recording disks which contain embedded servo tracks forprecise control of information positioning both for reading and forwriting operations.

It is yet another object of the present invention to provide anapparatus for writing information in multiple information tracks betweenembedded servo tracks.

It is a still further object of the present invention to provideapparatus and method for reading and writing information at high densityand with high information transmission rates.

It is also an object of the present invention to provide a method andsystem for precisely locating individual information bits or sets ofbits on a rotatable recording medium.

It is yet another object of the present invention to provide servo andfeedback control mechanisms which are capable of compensating for systemvariations such as disk flatness, bearing tolerances and motor speed.

It is also an object of the present invention to provide an apparatusand method in which information which is written onto a rotatingCD-ROM-like medium may be immediately read therefrom so as to be able todetermine its accuracy and to immediately apply error correction orerror compensation.

It is a still further object of the present invention to provide a servofeedback tracking apparatus which is capable of reading and writinginformation from a plurality of layers on a rotating CD-ROM-like medium.

It is a still further object of the present invention to provide anapparatus for multi-layer writing and for multi-layer reading ofrotatable information disks.

It is a still further object of the present invention to provide amechanism which is capable of determining variations in rotation speed,particularly those variations associated with end of motor life.

It is a still further object of the present invention to provide amethod and system for determining whether or not a laser semiconductorlight source is at or near its end of life point.

It is also an object of the present invention to employreading-after-writing capabilities using laser power modulation.

It is a still further object of the present invention to provide asystem for writing to and reading from a plurality of layers on a singledisk.

It is also an object of the present invention to provide a read/writehead which is particularly suitable for use with multi-layer trackedsystems in which different layers are employed on opposite sides of thesame disk.

It is a further object of the present invention to provide a system forprecisely tracking the position of a laser beam so that information maybe written to and read from a rotatable disk in a reliable, dense,closely packed fashion with a low access time and with a hightransmission rate.

It is a object of the present invention to provide a writableinformation storage system using a rotatable medium which is compatiblewith a variety of different data formats.

It is a still further object of the present invention to provide aninformation storage medium in which the information is stored in avolume as opposed to merely storing information in a single plane.

It is also an object of the present invention to provide a blankrecording disk which includes either one or a plurality of layers forwriting and reading operations.

It is yet another object of the present invention to provide a writableblank information medium containing a plurality of servo tracks forprecise positioning of information storage bits on opposite sides of arotatable disk.

It is yet another object of the present invention to take advantage offrequency doubling and phase characteristics of reflected trackingsignals which result from a beam of light being dithered across an edgeof optically different materials.

It is a still further object of the present invention to provide an areaon an information disk which includes information describing locationswhich are in error.

It is yet another object of the present invention to provide nearlysimultaneous writing and read-back capabilities.

It is a still further object of the present invention to provide signalprocessing circuitry for determining laser beam position and also fordetermining a layer in a rotatable recording medium which is beingemployed for reading or writing operations.

It is a still further object of the present invention to provide aninexpensive recording medium which is particularly useful in write-onceapplications.

It is an object of the present invention to provide a system and methodfor continuous spiral track edge following.

It is also an object of the present invention to create athree-dimensional bit map of information on a recording medium so as toimprove access time.

It is yet another object of the present invention to integrate errordetection capabilities in an information storage system involvingrotation and precision bit mapping of storage locations.

It is also an object of the present invention to provide simultaneousservo tracking of laser focus within a given layer of a multi-layer disktogether with servo tracking along an edge.

It is also an object of the present invention to be able to provideimproved measurement for rotation speed of recordable disk media.

It is yet another object of the present invention to be able to create arecorded disk information storage medium having indications thereon ofactual recording parameters such as physical speed, spacing, format andangular velocity.

It is a further object of the present invention to more accuratelydetermine angular rotation velocity by more accurately determiningradial position.

It is a still further object of the present invention to be able tostore rotational velocity information over time so as to provide anassessment of drive motor functioning particularly with respect to agingand/or end-of-life predictions and warnings.

Lastly, but not limited hereto, it is an object of the present inventionto provide systems, methods and apparatus for economical writing ofinformation on a rotatable disk using laser writing and positioncontrol.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is an is a partially isometric functional block diagramillustrating the basic components and operating principles behinddithering of laser light with respect to a compact-disk-like parts;

FIGS. 2A-2G illustrate steps employed in a process which produces blankrecording disks for use in conjunction with the system of laser writingused in the present invention;

FIG. 3 illustrates a cross-sectional view of a portion of a CD-ROM-likerecording disk particularly showing multiple layers in which informationmay be recorded;

FIG. 4 is a side elevation, cross-sectional view of a portion of aCD-ROM-like recording disk, similar to the view shown in FIG. 3, buthere more particularly illustrating the inclusion of multiple servotrack layers;

FIG. 5A is a top view (not to scale) illustrating the presence of aregistration mark and a spiral track pattern in which servo tracks arepreferably disposed;

FIG. 5B is a view similar to FIG. 5A but more particularly illustratinga radial reference (registration) mark extending across the entireradial extent of the disk;

FIG. 6 is a top view illustrating the relationship between servo trackwidth and the width of the medium portion which is employed forinformation storage;

FIG. 7 is a top elevation view illustrating the relationship betweenspots of laser light employed for tracking along a servo strip andcorresponding laser spots which are employed for reading and writinginformation into recordable material lying between servo track portions;

FIG. 8 is a view similar to FIG. 7 but more particularly illustratingthe situation in which information is written with respect to the rightside of the tracking spot as opposed to the left side which is shown inFIG. 7;

FIG. 9A is a graphical illustration of the relationship between lasertracking spot position and intensity of the resulting reflected signal;

FIG. 9B is a graphical illustration similar to FIG. 9A but moreparticularly showing the phase relationship of the resulting signal whenthe laser tracking spot is directed on the opposite edge of the embeddedservo track;

FIG. 10 is a functional block diagram illustrating an overall system oflaser light production and laser position control employing the servotracking system of the present invention;

FIG. 11 is a side elevation view illustrating positioning of a lens sothat its focal point lies in the proper plane;

FIG. 12 is a view, similar to FIG. 11, illustrating collimation of laserbeam output;

FIG. 13 is a functional block diagram illustrating a signal processingcircuit which is employed to provide position indication in a radialdirection;

FIG. 14 is a functional block diagram illustrating an alternateembodiment of a system in accordance with the present invention;

FIG. 15 is a functional block diagram illustrating a system inaccordance with the present invention in which laser power is modulatedto produce the desired writing and read-after-writing operations;

FIG. 16 is a plot of laser light power as a function of semiconductorlaser drive current;

FIG. 17 is a functional block diagram illustrating an embodiment of thepresent invention employing multi-frequency dithering;

FIG. 18 is a functional diagram illustrating an optical arrangementwhich is employable as a mechanism for reading and writing informationon both sides of a rotating information storage medium blank;

FIG. 19 is an optical path diagram more particularly illustrating anoverall optical system for reading and writing information to both sidesof a information storage blank;

FIG. 20 is a side elevation view of a read/write head which may beemployed in conjunction with the system in the present invention;

FIG. 21 is a partially isometric and partially functional block diagramillustrating a system in accordance with present invention in which animmediate read-after-write operation is performed;

FIG. 22 is a functional block diagram illustrating a system inaccordance with the present invention which is similar to the systemshown in FIG. 10 but more particularly illustrating the monitoringfunctions which may be provided relative to the laser aging process;

FIGS. 23A-23C illustrate yet another positional parameter which may becontrolled through the utilization of aspects of the present invention;

FIGS. 24A-24C illustrate output signal waveforms produced in the variousconfigurations states shown in FIGS. 23A-23C, respectively;

FIG. 25 illustrates a system in accordance with the present inventionwhich is similar to FIG. 15 but yet particularly illustrates a mechanismwhich may be employed to induce the desired variation shown in FIGS.23A-23C;

FIG. 26 is a plot which is representative of the modulation index for asemiconductor laser device as a function of time;

FIG. 27A is a side elevational view illustrating the operation of anedge-emitting laser diode device particularly showing light outputmonitoring device at the rear of the cell;

FIG. 27B is a side elevation view of three vertical channelsemiconductor emitting lasers arranged in an array;

FIG. 28 is a view similar to FIG. 15 but more particularly illustratingapparatus elements for determining rotation velocity, particularlyangular velocity, and for thus creating a recording machine havinginformation thereon indicative of physical speed, format, timing andspacing parameters;

FIG. 29 is a flow chart illustrating the steps in a method fordetermining angular velocity, particularly angular velocity, over aperiod of time to determine drive motor aging characteristics;

FIG. 30 is a flow chart illustrating in detail the step shown in FIG. 29relating to measuring linear velocity; and

FIG. 31 is a graph illustrating typical angular velocity measurements asa function of time for comparison to provide drive motor performance andaging indications.

DETAILED DESCRIPTION OF THE INVENTION

In a prior filed application, Ser. No. 08/626,302, filed Apr. 1, 1996,and issued on Nov. 4, 1997, as U.S. Pat. No. 5,684,782 which is herebyincorporated herein by reference, the generation and utilization of thesignal S×I was demonstrated. In particular, the inventors thereindiscerned that if a laser light source is dithered back and forth acrossan optical boundary exhibiting different optical properties, such asreflectivity, it is possible to employ the information present in themodulated returning light to determine and thus to control laserposition. In the present application, the inventors therein extend theirprior work to cover situations in which writing information to a laserdisk is a primary objective. In particular, the applicants hereindescribe a writing system based upon signal processing techniquesdemonstrated in several related prior patent applications submitted bythe same inventors as herein.

For a more complete understanding of the present invention, attention isdirected to FIG. 1 wherein there is shown, in schematic diagram form, anarrangement which may be employed to illustrate the operation of thepresent invention. In particular, in FIG. 1, there is shown diodeinjection laser 10 which directs laser light through preferred crossedfiber 80 which corrects for unequal divergence of the laser light intolens 100. Crossed filters 80 serve to circularize the laser light sothat the divergence in both directions is equal. The physicalpositioning of lens 100 is controlled by means of voice coil 20 which isprovided with an appropriate dithering signal from function generator60. This signal also includes an appropriate DC offset control level.Light from lens 100 is directed on to optical disk or CD target 30 fromwhich it is reflected back through lens 100 and diode injection laser 10to photodetector 40. The electrical signal from photodetector 40 may beemployed directly. Alternatively, it has been observed by the presentinventors that the drive current passing through diode injection laser10 is, in fact, modulated by the light returning from the target.Accordingly, by providing a resistor R (reference numeral 90) in thecurrent path of power supply 70, it is possible to extract this samesignal as a voltage drop across resistor R by connecting its oppositeends to differential amplifier 50 which provides an alternate outputsignal. By integrating this signal over an appropriate time period τ(measured in milliseconds), it is possible to produce an outputresultant signal, also referred to herein as R (not to be confused withthe resistance R), which is proportional to the amount by which the lensshould be repositioned in order to achieve optimal focus for aparticular layer.

For purposes of understanding the operation of the present invention,FIG. 1 should be understood to be a general schematic description forperforming the dithering operation on a lens or other optical elementthrough which the light from a laser source propagates. In preferredembodiments of the present invention, the laser light source comprises asemiconductor laser diode. Furthermore, for purposes of the presentinvention, the dithering function can be thought of as causing the laserlight to move back and forth in an oscillatory manner in either of twoways: (1) towards and away from the information medium target; and (2)back and forth across substantially (locally) parallel reference tracksin what is essentially a radial direction as seen with respect to target30 in FIG. 1. This oscillatory motion or dithering produces a particularsignal S×I which can be analyzed in the various manners as taught in theabove-referenced prior application. It is to be particularly noted thatanalysis of the signal S×I can produce very important information withrespect to positioning and control of the laser light source.

It is the precision of position control which permits the presentinvention to be useful in a number of different ways particularly withrespect to laser writing operations. In particular, in accordance withone aspect of the present invention, the dithering of the laser lightsource towards and away from the target medium provides a mechanism forprecision focus control of the laser light within the medium at variousdepths, that is, within selectable layers. With respect to multi-layeredmedia, the present invention, because of the precision of its focuscontrol, provides a mechanism for precisely determining the focal pointof the laser spot within various layers of the disk. This allowsmulti-layered disk read and write operations.

In another aspect of the present invention, the precise nature of thesignal analysis that is carried out by the present inventors providesinformation relevant to positional control in a radial direction. Thisis particularly useful in those situations where the information storedon the disk or other medium is arranged in a plurality of substantiallyparallel tracks (for purposes of the present invention, tracks areconsidered parallel even if curved into spirals or arranged asconcentric circles; parallelness is a local phenomenon). In accordancewith a preferred embodiment of the present invention, these tracks arearranged in a single concentric spiral. An alternative arrangement fordisk storage media employs information and reference tracks disposed onconcentric circular tracks at selected radii. In point of fact, thepattern is essentially arbitrary. Multiple spiral tracks are alsoemployable. The present invention also permits different layers withinthe medium to incorporate different track patterns: one layer could bearranged as a spiral while another is configured in concentric circles.Where desired, each layer in the medium is also provided with its ownreference datum mark so as to make it easier to access any part of thedisk (or other shape) storage medium via a complete mapping of itssurface (radially and angularly). The reference datum is positional sothat information on different layers of the disk is accessedsynchronously with each other.

As is demonstrated below, the signal processing aspects of the presentinvention permit precise control of the laser so that it may bepositioned in a proper focal plane (if needed or desired) and likewisepositioned at a known radial location. For purposes of writinginformation at desirably high densities, this precision is essential.

While the above-mentioned prior application was directed primarily toreading information from a disk, it should be appreciated, however, thatwriting information to a disk requires a much more precise controlmechanism. Furthermore, those skilled in the art fully appreciate thatthe mechanisms employed for writing and reading should permit asignificantly dense configuration of bit patterns on the informationstorage medium. In the present invention, the precision of control basedupon signal processing aspects provide the overall system with thenecessary degree of control which permits extremely dense writing ofinformation onto an impressionable medium.

If one is involved in the design of a system for writing information toa disk, one soon fully appreciates that it is not enough simply to writethe information to a disk, but, rather, that it is extremely importantthat the information be written onto those locations from which the datacan be retrieved. Again, it is the precision of control associated withthe signal processing methods described herein that provides therequired degree of precision writing control which is sufficient toachieve the necessary objective of being able to reliably retrieve theinformation from the recording medium at a later point in time.Additionally, the methods and apparatus described herein also provide anability to monitor the performance of laser writing source and to alsomonitor the quality of the medium itself.

In furtherance of achieving the writing and reading objectives of thepresent invention, a recording medium is employed on which there arealready present differentially reflective tracks for selected laserlight reflection. (Although differential reflectivity is the preferredapproach herein, it should be noted that any optical property differencemay be used.) These tracks are employed as a basis for positioning lasersources for both reading information from the medium and for writinginformation to the medium. Accordingly, as a first aspect of the claimedinvention, there is described a disk medium and a method of constructingit with tracks which are appropriate for the purposes of the presentinvention. It should be noted, in particular, that the design of thepresent invention fully exploits the capabilities of these specialdisks. These disks have also been particularly designed forconsiderations of cost and ease of fabrication. Additionally, thestructure of the disk recording media associated with the presentinvention has been structured so as to maximize, to the greatest extentpossible, the density of information which may be written to anyparticular layer in the disk. It should also be particularly noted thatthe fabrication methods set out herein are not only useable in theconstruction of single-layer recording media, but that they are alsoextended to the construction of multi-layer media.

For a complete understanding of the process involved in constructingrecordable media useable with the present invention, attention isdirected to FIGS. 2A-2G. These figures show various stages (30a-30g) ofconstruction of a single-layered version of a writable medium useful inthe present invention.

In the beginning of the process illustrated in FIGS. 2A-2G, it is notedthat one begins with disk substrate 30a in FIG. 2A comprising asufficiently rigid polymeric material such as a polycarbonate resin. Itshould be particularly borne in mind that the substrate material withwhich one starts the production of recordable media in accordance withthe present invention does not have to include material which isoptically flat. This is a decided advantage which results from thepresence of embedded tracks to make up for any deficiencies that mayoccur because the initial starting substrate is not in and of itselfnecessarily an optically flat piece of material. This clearly has costadvantages for those using the present system.

As shown in FIG. 2B, substrate 30a from FIG. 2A is coated with positivephotoresist 33. This is readily applied by any number of well knownmethods including spin coating which is preferred.

Once photoresist 33 is applied to initial substrate 31, the photoresistmaterial is exposed through a mask which, in preferred embodiments,includes a spiral pattern. While preferred embodiments of the presentinvention employ tracks having a spiral geometry, any convenient trackpattern is useable including concentrically arranged circles or multiple(that is, interleaved) spirals. In practice, it is noted that the maskthat is employed for creating the pattern seen in FIG. 2C whenphotoresist 33 is exposed is preferably not a freestanding mask but,rather, is typically a structure which is mounted on a separate rigidtransparent material such as quartz.

FIG. 2C illustrates resultant structure 30c which results whenphotoresist 33 is exposed through a desired mask and then developed. Theresulting photoresist pattern 33 is actually the negative (in thephotographic sense) of the desired track pattern.

It is noted that metal layer 34' is applied to structure 30c. Inpreferred embodiments of the present invention, this metallic layercomprises aluminum which is approximately 150 to 200 Angstroms thick. Itis particularly noted that while aluminum is a preferable material,particularly because it is inexpensive yet effective, other materialssuch as gold or reflective polymers. In this regard, it is noted thatthe primarily desirable property for layer 34' is that, in finishedproduct form, it provides a layer from which light is reflected in adifferent manner than it is from the rest of the substrate material.Accordingly, the present invention finds it very desirable to employmaterials having different reflectivities. It is this difference inreflectivity which produces signals from which the present applicantshave been able to extract useful positioning information. The othernecessary property for layer 34 is that, upon deposition, it adheres tosubstrate 30 and, furthermore, that, during the photoresist removalprocess, it does not operate to interfere with photoresist removal.Also, as noted above, optical properties other than reflectivity may beused.

The result of photoresist removal, namely, the structure shown in FIG.2E as substrate 30e, illustrates the result of a wet chemical etch inthe photoresist removal process. Typically, photoresist material isremoved by a wet chemical etch followed by a cleaning process forremoving any residue. The deposited reflective layer 34 which had, inFIG. 2D, been shown as being attached to patterned photoresist layer 33is now removed in those locations for which this attachment occurredalong with the underlying photoresist. It should be noted that the stepsillustrated in FIGS. 2A-2E are ones which readily lend themselves tomass photolithographic production techniques. This and subsequenttechniques which are also employable in mass production fashion operatecooperatively to keep the cost of production low while at the same timenot interfering with the speed at which production can occur.

Attention is next directed to the process step illustrated in FIG. 2F inwhich semi-transparent layer 35 is applied to structure 30e (FIG. 2E).Layer 35 preferably comprises a layer such as polymethylmethacrylate(PMMA) or an equivalent. In preferred embodiments of the presentinvention, this material is spin coated onto structure 30e. It is notedthat at this juncture an optional planarization step may be performed.

It is also important to note that structure 30f, shown in FIG. 2F, is animportant structure in that it is the starting point for the depositionof further layers if a multi-layer structure is desired. Thesesubsequent layers may or may not include imbedded track material 34. Iftracks are imbedded in subsequent layers, it may be of a differentmaterial than that found in lower layers, and in particular, it may havedifferent optical transmissivity properties or use an entirely differentoptical property.

It is also noted that the process for embedded track creation,illustrated in FIGS. 2A-2G, is fully compatible with providing differentinformation formats in different layers. These formats include theCD-ROM format and the DVD format.

Whether one employs a single-layer or a multi-layer structure, the lastlayer preferably includes layer 36 which preferably comprises a layer ofaluminum which is approximately 50 Angstroms thick for purposes ofproviding differential reflectivity, logo printing, etc. This ispreferably deposited by flash coating. A resultant single layerrecordable medium is therefore shown as structure 30g in FIG. 2G.

It is to be noted in the present invention that it is layer 35, thepolymethylmethacrylate layer or its equivalent, which is employed forthe purpose of providing information storing regions. More particularly,these regions are located between tracks 34.

It should also be appreciated by those of ordinary skill in thephotolithic arts that the process described above is merely illustrativeof a number of different possible processes which may be employed toproduce the structure shown in FIG. 2G. In particular, it is noted thatthe process described employs the utilization of a negative photo mask.Other methods and processes could readily employ positive (in thephotographic sense) masks. Processes based on selective deposition,rather than selective removal, are also employable.

It is noted that, with respect to FIGS. 2A-2G, representativecross-sections of the desired recordable media are shown at variousstages. In terms of dimensions, however, these figures are not drawn toscale. Nonetheless, the representative relative layer heights asdescribed above are shown.

With specific reference to the situation in which a multi-layered diskis produced, attention is directed to FIG. 3. As pointed out above inthe stage shown in FIG. 2F, one may again coat the substrate with alayer of recordable material such as polymethylmethacrylate and repeatfor as many layers as are desired and are practicable. For example, athree-layer medium is illustrated in FIG. 3. In this cross-sectionaldiagram, recordable layers 35a, 35b and 35c are shown. Additionally, itis noted that each one of these layers is logically referenced to track34. It is these tracks which provide a volume reference for the entiresolid structure shown in FIG. 3. In particular, it is to be specificallynoted that each of the three recordable layers is referenced to thetrack(s) in the first layer (layer 35a). Thus, tracks provided forwriting and reading control are employed as a reference structure forthe entire volume of the disk. This feature is particularly important tonote for those situations in which three-dimensional volume hologramsare employed in conjunction with the present invention; accordingly, itis noted that the utilization of the precision tracking aspects of thepresent invention are entirely consistent with utilization ofholographic storage and holographic storage media. The fact that allthree layers shown in FIG. 3 can be referenced to the same set of tracks34 means that the information in all of the layers shown may be writtenor read using a single reference. This has a particular advantage inthat data synchronization and storage between and amongst the variouslayers is readily achieved. Accordingly, in the structures of thepresent invention, particularly that which is illustrated in FIG. 3, itis much easier to maintain logical connections of data present inmulti-layers. For example, in those instances where the informationstored in different layers represents audio portions of the informationin different languages, it is readily seen that it is extremely easy tomaintain synchronization between the various aspects of this informationbecause each of the layers is referenced to a common set of tracks.

However, it is noted that the present invention may also employreflective tracks 34 in multiple layers. Such a structure is illustratedin FIG. 4 which is particularly useful in those situations where onewishes to write and/or to read from opposite sides of the disk platter.These considerations are more fully discussed below with specificreference to FIGS. 18 and 19. However, at this juncture, it is to beparticularly noted that the structure illustrated in FIG. 4 isparticularly appropriate for those circumstances in which one wishes toread simultaneously from both sides of the disk or to write to one sideand to read from the other one. This is also particularly desirable inthose situations where the data on the different sides is unconnected orunrelated and/or in those situations in which the maximum amount of datatransferred in a given instant of time is to be provided. Theconstruction of the structure shown in FIG. 4 proceeds in the samemanner as the construction of the structure shown in FIG. 3 with theexception that, before deposition of a second information-bearing layer,a layer of photoresist is applied, exposed and developed and is removedafter deposition of a suitable reflective material, such as aluminum,for use as tracks 34a and (separate) tracks 34b in FIG. 4.

Attention is now directed to those aspects of the present inventionwhich are illustrated in FIG. 5A. In particular, FIG. 5A illustrates atop view of a recording medium which is constructed in consonance withthe present invention. It is, however, noted that the number of spiraltracks shown (one) and the distances between the spiral tracks areemployed for illustrative purposes only. In point of fact, there aremany thousands of tracks per radial inch and they are disposed much moreclosely together than is shown. However, in this regard, FIG. 5A isemployed for illustrative purposes only.

The most important aspects of FIG. 5A are discussed first. Inparticular, it is noted that recordable medium disk 30 has imbeddedtherein (preferably spiral) tracks 34. It is across the edges of thesetracks that a laser light is moved in a periodic, oscillatory fashion toprovide one form of the desired tracking in the present invention.Frequency doubling and its associated signal processing aspects,particularly including special digital signal processing aspects, areemployed to achieve the desired degree of laser beam position controlfor both reading and writing operations. However, it is noted that it isthe writing operations that are primarily (though not exclusively) ofinterest in the present application.

One of the important aspects to appreciate from FIG. 5A is that thereare shown a plurality of typical laser spot focus positions 38 disposedalong the outer edge of track 34. It is noted that the utilization of aspiral track pattern enables continuous edge-following. Thisedge-following can begin as early as outer registration mark 37 andcontinue all the way into the central region of the disk, typicallyspaced some distance apart from spindle opening 41 in disk 30. As shownin FIG. 5B, a registration mark 37' may extend across several tracks andmay in fact extend across all the tracks to provide a reference forposition determination in the angular or θ- direction.

The tracking of a spot of laser light along an outer edge of a track isimportant for fully appreciating the difference between the presentinvention and certain other exemplary systems that could employtrack-following methods. In some of these other methods, tracking isaccomplished by making sure that the laser spot remains in a location"somewhere between" locally parallel tracks. Thus, in other systems, thespot itself may be present at any one of a plurality of positionsbetween the tracks.

Nonetheless, it is a significant feature of the present invention thattrack edge-following is employed in special ways. Not only does thepresent invention employ track edge-following for purposes of providingsuperb precision and control of laser spot placement, it is seen thatthis is done in a continuous fashion. Thus, in the present invention,when information is written to a disk, this information may be writtenin continuous fashion from beginning to end in a manner which assuresvery precise positioning of the stored information.

It is furthermore seen in the discussions below that, not only does theedge-following technique employed in the present invention producessuperior results, it is in fact coupled with a digital signal processingapproach which in fact enables multiple information tracks to be writtento and read from a plurality of spiral information tracks which runparallel to the tracks 34. In particular, it is seen in the presentinvention that at least four separate information tracks may be providedbetween track edges. The present invention, however, provides accurateand precise means for identifying the location of a laser spot forreading or writing so that information is written to any reasonablenumber of information tracks which lie between reference tracks 34, asprovided herein. This aspect of the present invention permits anincreased level of information storage density and, furthermore, resultsin the more precise and accurate writing and retrieval of storedinformation.

It is noted that a large number of advantages of the present inventionarise out of the fact that the present applicants exploit certain signalprocessing attributes associated with dithering the laser light signalacross a reference track edge. In particular, analysis by the presentinventors of the resultant output signal provides a mechanism not onlyfor determining when a laser spot is focused on a particular track edge,but they also employ the same fundamental signal processing method todetermine the actual distance that the spot is displaced with referenceto the track edge. Furthermore, analysis of the signals produced by themethod employed in the present invention yields specific informationwhich allows the user (and/or system employed in conjunction with thepresent invention) to determine which side of the track the laser spotis focused onto. That is, the system of the present invention is readilycapable of determining whether or not the laser spot is focused on aninside edge or on an outside edge of any given track. Furthermore, it isalso possible, by using the present invention, its circuits, apparatusand methodology, to determine exactly how far from an edge that thelaser light is focused. This provides an unprecedented degree of controlfor laser focal spot positioning. Furthermore, as pointed out above,this is to be contrasted with other systems in which the laser spot isallowed to drift from one track to the next in a sort of bang-bangcontrol system fashion. These other systems only assure that the laserspot is somewhere between tracks. This methodology is clearly deficientin terms of providing optimal information storage density particularlysince any inter-track gap can include only one information track perlayer.

For the reasons pointed out above, it is therefore important to observethat focusing of laser light onto disk 30 occurs at points 38 which areshown on the outside of track 34. That is, for purposes of illustratingcontinuous edge following, spots of laser light are shown as beingpositioned on the outside of the edge of track 34. It should be notedthat spindle opening 41, while present in FIG. 5A, is not shown in FIGS.2, 3 or 4 for the simple reason that these figures merely illustratetypical cross-sectional areas through disk 30 which do not and/or wouldnot include spindle opening 41. (It is noted that the "spots" shown inFIG. 5A are merely illustrative and are not meant to suggest either thatthe laser is necessarily pulsed or that the spot size is drawn toscale.)

Further appreciation of the scale and operation of the present inventionmay, however, be gleaned from consideration of the illustration shown inFIG. 6. FIG. 6 represents an enlarged view of the situation shown at amacroscopic scale in FIG. 5A. In particular, FIG. 6 illustrates some ofthe spacing aspects of the present invention in terms of the increasedability for information storage in the present invention. FIG. 6illustrates an area of disk 30 which includes two adjacent portions ofspiral track 34. While these spiral track portions extend in a curvedfashion around disk 30, nonetheless, in a local microscopic view, trackportions 34 are substantially parallel even though curved. In thepresent invention, the actual width of the track and the space betweentracks and the size of optical spot 38 varies depending upon systemdesign constraints, trade-offs and desired disk format, be it CD-ROM,DVD, etc.

In particular, it is noted in FIG. 6 that if one designates the width ofa track 34 as X, it is possible to employ inter-track spacing which isapproximately 2X to 4X in width. This spacing permits from 2 to 4 tracksof information to be written in the recordable medium between adjacenttrack portions. This is in stark contrast to the single informationtrack which could be employed in other systems.

One of the items that is evident from the prior patent application whichis incorporated herein by reference is that the system and method of thepresent invention enables one to not only follow the edge of a desiredtrack but, also, enables one to position a spot of laser light in avariety of positions with respect to the edge of a track. These aspectsare particularly illustrated in FIGS. 7, 8 and 9. In particular, FIG. 9below illustrates some of the signal processing aspects associated withthe present invention. However, FIGS. 7 and 8 more particularlyillustrate how the track-following method of the present invention maybe employed to write four separate information tracks in between tracks34.

For example, in FIG. 7, it is seen that tracking spot 38a tracks the"left edge" of track 34". This tracking permits information writing orreading spot 39a to be positioned approximately 3.5X to the left oftracking spot 38a. Laser beam spot 39a can be used either reading orwriting as appropriate over different periods of time. By physicallycoupling the spacing between laser spots 38a and 39a, it is seen that atrack of information may be written at a distance which is approximatelyX/2 from the right edge of track 34'. By moving tracking spot 38aslightly to the right so that it tracks along the inside of the leftedge of track 34", it is seen that an information track may be writtenwith laser spot 39b. In this regard, it is particularly important torealize that the system and method of the present invention provides amechanism for determining which side of track 34" the laser spot 38 istracking. In a similar manner by moving laser spot 38b to the positionillustrated by laser spot 38c, it is possible to write an informationtrack beneath spot 39c. Again, it is noted that, by tracking alongeither the left or right edge of track 34", it is possible to write upto four information tracks between each and every reflective referencetrack 34.

As described above, however, the present invention is not limited to thecase in which the ratio between track width and inter-track spacing is1:4. However, FIG. 7 is nonetheless illustrative of typical gains ininformation storage density which are achieved by the present invention.In particular, as mentioned above, the present invention provides adefinitive indication not only of the specific edge of a track which isbeing followed, but it also provides an exact indication of the distancebetween the tracking spot and an edge. As a result, information storagedensity is limited in a significant way only by the size of the laserspot which may be focused. However, even here, focusing aspects of thepresent invention in which dithering towards and away from therecordable medium is permitted provides even tighter control of thelaser focus spot as it appears in any selected or desired layer of thedisk. Accordingly, it is seen that the inter-track spacing of 4X shownin FIG. 7 is illustrative only and that spacings of 5X, 6X and beyondare nonetheless possible with the present invention thus providing foreven greater recording density in terms of tracks per inch orinformation bits per square area. (Tracks 34' and 34" in FIG. 7 areshown as straight lines only for convenience and ease of understanding.)

With respect to FIG. 8A, it illustrates yet another aspect of thepresent invention, namely, the fact that, in contrast to the situationin FIG. 7 where tracking spots lie to the right of information readingand writing spots, the opposite is true in FIG. 8. In particular, inFIG. 8, reading and writing spots 39a-39d are shown to be to the rightof tracking spots 38a-38d respectively. Furthermore, it is to beparticularly noted that the writing of information tracks betweenreference tracks 34' and 34" may in fact be determined solely byreference to tracking spots moving along the interior edges shown inFIG. 8, namely, the right edge of reference track 34' and the left edgeof reference track 34". In such a situation, reference trackings spots38a and 38b would be located in corresponding positions along the leftedge of track 34". Any of these relationships between tracking spot andinformation spot may be employed in keeping within the parameters, scopeand specifications of the present invention.

Important aspects of the present invention are discernable from theillustrations shown in FIGS. 9A and 9B. In particular, these two figuresillustrate the mechanism by which the present invention is employed todiscern which edge of a track is being followed. This is essential foran understanding of the operation of threshold detector 81 in FIG. 13discussed more particularly below. With specific reference to FIG. 9A,it is noted that the spot shown is being dithered back and forth acrossthe edge of a track and appears, during the dithering operation, in oneof three positions (1, 2 and 3). Since positions 1 and 3 are the samewith respect to the track edge, the resulting intensity of the lightreflected back is the same for these two positions. This is shown by theheights of the detected signal coming back as being the same in thegraph which forms the right portion of FIG. 9A. It is seen that, becauseof the difference in reflectivity, the intensity of the returning lightvaries in substantially the fashion shown. It is minimal when the spotis in the position indicated by dotted lines 2 in FIG. 9A. In theoperation of the present invention, the laser light tracking signalreflected back from the recording medium (or the play back medium) isconverted to an electrical signal by various means. For ease ofunderstanding the fundamental aspects of the present invention, oneshould envision the returning (i.e., reflected) laser tracking signal asbeing directed to a photodetector which converts the light to a varyingelectrical signal. This signal is shown in the right hand portions ofFIGS. 9A and 9B. However, it is to be particularly noted in FIG. 9A thatdithering of the spot between the positions shown never results in asituation which produces maximum reflectivity such as that which occursin FIG. 9B when the tracking spot is entirely outside the boundary ofthe track edge which is being followed. Rather, in the case illustratedin FIG. 9B in position 2 (shown by a dotted line circle), there resultsa peak or maximum value for the electrical signal which is produced. Inaccordance with the principles of the present invention, the relativephase difference between the signals shown in FIGS. 9A and 9B is readilyseen to be employable to determine which track edge is being followed,namely, the left edge in FIG. 9A or the right edge in FIG. 9B. Aconvenient threshold value T₁ is easily determined and employed as onemechanism for discerning differences between signals from FIG. 9A fromcorresponding signals from FIG. 9B. It is important to note that, in thepractice of the present invention, it is very desirable, particularly incertain embodiments, to be able to determine on which side of the trackthe laser is focused. This information is, therefore, seen to becorrespondingly important for determining, in both relative and absolutefashions, exactly where one is with respect to reference tracks on thedisk and also with respect to other potential fixed reference points onthe disk including things such as registration mark 37.

Next is considered the embodiment of the present invention which isillustrated in FIG. 10. With specific reference to FIG. 10, it isimportant to note that certain embodiments of the present inventionemploy the laser diode source in a second role, namely, in the role ofsignal detector. However, with specific reference to FIG. 10, it isnoted that the embodiments shown therein are ones in which separatephotodetectors are provided. For a discussion of embodiments of thepresent invention in which the laser source is also used as a detector,attention is directed to FIG. 17 and to the discussions below concerningthis figure. However, since FIG. 10 does not specifically include thisparticular feature, it should be observed that instead, in theembodiment shown in FIG. 10, there are included separate photodetectors68 and 69 which function as mechanisms for controlling focus in adesired layer and for controlling position with respect to a referencetrack, respectively.

In the embodiment shown in FIG. 10, laser light source 65 is controlledin two different directions by means of position control circuit 64.These control circuits are more particularly described above and inprior application Ser. No. 08/626,302 filed Apr. 1, 1996, which has beenincorporated herein by reference as mentioned above. In particular, withreference to the situation in which the recording medium is a rotatingdisk having reference tracks, it is noted that position control circuit64 is used to position laser source 65 in a radial direction withrespect to one or more edges of the embedded reference tracks. In asimilar fashion, the signal from layer detector filter 62 providesposition control information for achieving focus of the light from laser65 within a specific layer (35a-35c) of multi-layer medium 30. Focuscontrol is provided either by moving laser light source 65 in adirection towards and away from medium 30 (the Z-direction) or byvarying the focus of lens 72.

Laser light from source 65 is directed to a first beam splitter 67 whichdirects a first beam to medium 30 through lens 72 and beam splitter 73.As described above with respect to controlling focus so that focus lieswithin a particular layer, dithering at a frequency ω_(R) is provided sothat either lens 72 or laser source 65 are in effect moved towards andaway from medium 30. However, it is noted that, in preferred embodimentsof the present invention, dither modulation is preferably achievedeither by controlling the focal length of lens 72 in a mechanicalfashion or in an electromechanical fashion or by changing the focalposition of lens 72 with respect to medium 30. This is consideredsimpler than dither modulation achieved by moving laser source 65.

A second beam from beam splitter 67 is directed to reflective means 66which is likewise dither modulated (that is, moved oscillatorilly) at adifferent frequency ω_(T). This dither modulation is provided by anyconvenient means such as by use of a piezoelectric element attached to amirror or by an electromechanical device. In contrast to ditheringproduced by modulator 61, the dithering achieved by modulator 74 such asto move a beam of light backwards and forth across an edge of referencetrack 34 in medium 30 for purposes of providing focused control of thisreference beam, it is preferably directed through lens system 71 asshown in FIG. 10.

An important aspect of note in FIG. 10 is the fact that dithering at twodistinct frequencies ω_(R) and ω_(T) is employed. Dithering at thefrequency ω_(R) is employed and operates to provide control of focus sothat the focus is in fact maintained in a desired layer within medium30, even if there is only one layer. In a similar fashion, the ditheringat frequency ω_(T) is employed so as to provide position control in aradial direction for spinning disk media. Dithering at frequency ω_(T)is used to control positioning of laser light source 65 in a radialdirection.

It is noted that the preferred embodiments of the present inventionemploy semiconductor laser diodes as the laser light source. It is to benoted that this particular embodiment is desirable though not necessaryin the system shown in FIG. 10. In those systems of the presentinvention in which the laser light source operates both as a source anddetector, the use of semiconductor laser diodes or similar sources isrequired. However, when it is not specifically desirable to utilize thedrive current variant phenomenon that occurs when laser light isdirected back into the semiconductor laser, separate photodetectors areemployable. In these circumstances, the photodetectors may be of anyconvenient type in these circumstances.

In order to segregate desired feedback control signals, the apparatusshown in FIG. 10 employs filters 62 and 63, each of which may bedesigned as a band pass filter centered about the specific controldithering frequency (ω_(R) for focus control and ω_(T) for positioncontrol with respect to embedded reference tracks in medium 30). Usingthe signals from photodetectors 68 and 69, position detection andcontrol is provided in accordance with the descriptions found above. Inparticular, it is noted that position control may be achieved in eitheran analog or in a digital fashion with the digital fashion beingpreferred.

FIG. 11 illustrates a small portion of the apparatus shown in FIG. 10.However, and more importantly, it illustrates the notion of being ableto adjust either the position of lens 72 or the focal length of lens 72for purposes of initially determining and, subsequently, for controllingthe position of the focal waist plane which is the point of narrowestfocus for light beams 72 traveling parallel to the central axis of lens72. Thus, focus position within either layer 35a or 35b is determinedand, in fact, controlled by the use of a deformable lens or lenspositioning mechanism.

FIG. 12 illustrates the lens system which is specifically employed fordirecting a tracking beam onto reference tracks 34. In a multilayerdisk, it is necessary for the light beam to pass through several layersof the disk to read the tracking marks.

The present invention employs a compound lens system which keeps thebeam collimated over a longer distance so that it can reach the servolayer, rather than a simple lens which delivers light to the data layer.The compound lens keeps the tracking beam from diverging too soon withinthe disk.

Next is considered the specific circuit shown in FIG. 13. In thisregard, it is noted that an optimal understanding of the function of thecircuit shown in FIG. 13 is best comprehended from consideration ofFIGS. 9A and 9B. There are in fact two principle operations which areperformed by the circuit shown in FIG. 13. The first and primaryfunction is to determine which of the two sides of a given referencetrack the tracking beam is focused upon. In particular, for spiral orconcentric tracks on a disk, the beam may be focused on an inner edge(or left track edge as seen in FIG. 6) or an outer edge (or right trackedge as also seen in FIG. 6). These two situations are shown in FIGS. 9Aand 9B, respectively, (in particular, the differences are shown in theleft-hand portion of FIGS. 9A and 9B). The signal S×I, as seen in FIG.1, is first passed through threshold detector 81 which employs a valueT₁, as shown in the right-hand portions of FIGS. 9A and 9B, to determinewhether or not the spot is tracking along an inner or outer (left orright) edge. If one were reading or writing from only a singleinformation track disposed between reference tracks, threshold detector81 would suffice for providing a reference signal. However, asillustrated in FIGS. 7 or 8, it is possible to employ multipleinformation tracks between reference tracks. In such cases, it isnecessary to know more specific information about the position of thelaser spot. In such cases, it is necessary to know the specific signalvalues at the point 1, 2 and 3 as labelled in the right-half portion ofFIGS. 9A and 9B. While detector 81 can essentially determine the phaseof the signal S×I using the threshold value T₁, as seen in FIGS. 9A and9B, it is seen that more information with respect to specific signalintensity levels are needed to determine on which of the four positionsshown in FIGS. 9A and 9B the spot is located. In this regard, it shouldalso be particularly noted that, while detector 81 is described hereinas being a threshold detector, it is also possible to implement detector81 as a phase difference detector which determines the fact that thesignals shown in FIGS. 9A and 9B are respectively 180° out of phase withrespect to one another. Waveform detector 82 functions to discriminatebetween the two waveforms shown in FIGS. 9A and 9B irrespective ofvertical DC shifts in the signal patterns. The output signal fromwaveform detector 82 is supplied to up-down counter 83. This counterresponds to changes in detected laser spot position as seen by waveformdetector 82 and threshold 81. Thus, as the central focal spot of thetracking beam is moved across the edges of sequential reference tracks,a counter is provided which indicates a binary number which isessentially an information track address. In preferred embodiments ofthe present invention, this counter value/address is supplied to counterlogic block 84 which includes tables therein which map specific binarycounts to provide a radial position indicator signal. Thus, eachinformation track on the medium is provided with a reference address towhich the reading or writing head may be immediately directed.Accordingly, the circuit shown in FIG. 13 provides important advantagesin systems employing the present invention. In particular, it provides amechanism for addressing individual track locations on a medium such asa rotatable disk. In particular, it provides addressability in a radialdirection. The other aspect of addressability described herein relatesto addressability in the θ-direction. This aspect of the invention isdescribed in more detail below. For the moment, it is sufficient toappreciate that motion of the tracking spot across reference track edgesproduces electronic signal indications which describe not only whichside of a track on which one is located, but also provides an indicationof the specific distance away from the edge of a reference track forpurposes of specifying a definite one of a plurality of informationtracks located between reference tracks. For purposes of illustrationherein, it is noted that the system described shows four informationtracks between reference tracks. This provides essentially a 4 to 1advantage over other schemes which employ only a single informationtrack between two adjacent reference track portions.

Attention is next directed to the exemplary system shown in FIG. 14. Thesystem shown therein is provided for the purpose of writing informationonto a recordable medium. In particular, in the embodiment shown, thelaser light source is not specifically employed as a detector. The roleof the photodetector in the embodiment shown is provided by separatephotodetector 84 which receives light from beam splitter 86 whichdivides the laser light output from source 83 into a first beam forwriting onto select layers within medium 30 and a second beam which issupplied to ditherable reflective means 85. As described above,reflective means 85 preferably comprises a mirror to which is attached avibratable mechanical element such as a piezoelectric device. Thiselement provides a dithering action for the tracking beam which isreflected back and forth across an edge of reference track 34 which, asdescribed above, is preferably disposed in a spiral pattern on arotatable disk which is capable of being modified by action of impinginglaser light for the purpose of information storage.

It is to be particularly noted that, in accordance with the principlesof the present invention, such media include the typical CD-ROM disk andadditionally include such systems as magneto optic disks. The principlerequirements are that the medium include a reference track havingdifferential reflective properties and that the medium itself be capableof storing information by operation of laser light impingement.

In operation, the apparatus shown in FIG. 14 writes information bymodulating, via laser power control 91 which determines laser drivecurrent and thusly, the output power of laser source 83. Clearly, onerequires a relatively high intensity beam for writing purposes.Typically, in normal operations associated with compact disk informationstorage, the ratio between laser power for writing and laser power forreading is approximately 10 to 1, respectively. Clearly, it is notnecessary for full laser power to be supplied to the tracking beam whichis directed to ditherable reflector 85. In this regard, it is noted thatbeam splitters, such as beam splitter 86, are readily available in whichvarious amounts of laser power are directed in differing directions. Forpurposes of tracking, a signal of only a few percent of maximum laserstrength is required. Nonetheless, because of the particular arrangementprovided in FIG. 14, it is very desirable that photodetector 84,together with any corresponding electrical circuitry for amplification,detection or conditioning, exhibit an appropriate dynamic range since,during write operations, a greater than average level of laser power isdirected via beam splitter 86 to photodetector 84. Because the ditheringprovided to reflective means 85 is at a specific and known frequency,the signal which is analyzed via photodetector 84 only needs to beconsidered for its information content in the spectral region in thevicinity of ω_(T), the frequency at which dithering modulation isapplied to reflective means 85. As described above, the resulting signalS×I is analyzed by a dither tracking detector circuit such as that seenin FIG. 13 (or in the application incorporated herein by reference).This information is supplied to laser position control 81 which adjuststhe position of laser light source 83 in a radial direction. Forpurposes of topic focusing, simplicity and ease of understanding, thesystem shown in FIG. 14 is not illustrated with feedback loop controlmeans in place for laser focus control. Such a feedback loop is,however, illustrated in the method described above and below withspecific reference to FIGS. 10 and 17.

During write operations performed by the apparatus shown in FIG. 14,data supplied to laser power control 91 is used to vary the drivecurrent in a device such as a semiconductor laser diode. Modulated laserlight from source 83 is supplied, via beam splitter 86, to medium 30 onwhich it impinges to cause the writing operation. The writing operationoccurs at a distance D from the point of reference on track 34 which isused as a point of reference for precision writing. The value D rangestypically from approximately tens of microns to about 1 mm.

It is to be noted that proper operation of the apparatus shown in FIG.14 is possible because photodetector 84 and dither tracking detectorcircuit 82 may be employed to detect only those information signals inthe vicinity of ω_(T), the frequency at which reflective means 85 isdithered. Thus, tracking information is very easy to select out from anyother signals impinging upon detector 84. It is also to be noted thatthe embodiment shown in FIG. 14 is particularly desirable and usefulinsofar as the operation of the device shown in FIG. 14 is concerned; inparticular, it is noted that dense writing operations of multiple tracksof information between reference tracks is made possible because of theprecision provided by the tracking method described herein whosetheoretical basis is described above with respect to FIGS. 9A and 9B.

With respect to most of the embodiments described herein, it is veryimportant to note that precision of information position control is muchmore critical in operations involving writing than in those operationsinvolving reading. Write operations create a permanent or semi-permanentchange in the recording medium. If this information is to besubsequently read in a reliable and efficient manner, it is extremelyimportant that writing consistency be present in the writing system.Accordingly, one will find that this positional precision is a hallmarkof the systems described herein.

Attention is next directed to the specific embodiment illustrated inFIG. 15. This embodiment is particularly directed to a system and methodfor information writing. The system shown permits a string of data bitsentering laser power control unit 91 to be written into a recordablelayer on disk 30. As with all embodiments of the present invention, disk30 includes layer 34 which is employed for reference and trackingpurposes. Laser 83 provides a light signal both for reading and forwriting purposes. Laser power control unit 91 determines the level ofdrive current for laser 83 thus determining whether or not informationis to be read from or written onto the disk storage medium. Typically,there is a ratio of approximately 10 to 1 between the level of laserpower desirable for writing information in comparison with the levelwhich is desirable for reading information. Accordingly, this dynamicrange of laser power signals should be accommodated by photo detectors93 and 94.

Light from laser 83 is first provided to beam splitter 86 which passeson a portion of the light to beam splitter 92. A smaller fraction of thelaser power is directed towards reflective means or mirror 85 which inturn directs laser light from the tracking beam onto reference track 34.This mirror is dithered in the same fashion as shown for mirror 66 inFIG. 10. This dithering operation is employed in accordance with thetracking principles of the present invention and, in fact, returnsreflected light from the disk storage medium to mirror 85 and, thence,to splitter 86 and to photodetector 94 which supplies the resultingelectrical signal to tracking detector 95. An appropriate circuit fortracking detector 95 is shown in FIG. 13.

The portion of laser light signal from laser 83 which is not directed tomirror 85 is instead directed to beam splitter 92. The emitted laserlight is thus directed from beam splitter 92 to a particular one of thelayers in the recording medium. During read operations which arecontrolled by the read/write control signal line which is supplied toEnable Switch 96, the laser power is set on its lower (reading) levelpower and switch 96 is enabled to pass reflected photodetected signalsto data detector 97 which supplies the output signal. Photodetector 93senses the variation in reflection caused by previously written physicalvariations in a selected layer of the recording medium. During writeoperations, the read/write control signal line turns the enable switchoff so that data detector 97 is not fooled into believing that a datasignal is present at its input. At the same time, the read/write controlsignal line, either indirectly through enable switch 96 as shown or asdirectly controlling laser power control 91, causes the laser poweroutput to be increased to the appropriate level for writing informationonto rotating disk 30. In the embodiment shown in FIG. 15, reading andwriting operations are not carried on simultaneously but, rather, arespaced apart in time.

FIG. 16 is included and is discussed at this time in conjunction withthe above discussion with respect to the system shown in FIG. 15 whichis used for writing. In particular, FIG. 16 illustrates the fact thatthe power in a semiconductor laser beam is a function of semiconductordevice drive current. In particular, it illustrates the fact that thereis generally a current threshold, Icrit, below which lasing action doesnot start or is ineffectual. However, once a threshold current level isreached, the power in the laser beam is increased approximately linearlywith increasing levels of current. While not specifically illustrated inFIG. 16, it is noted that, generally as semiconductor laser devices age,the curve shown shifts further out to the right. This means that, as atypical semiconductor laser device ages, the amount of power present inthe laser beam decreases. Nonetheless, it should also be appreciatedfrom FIG. 16, including those versions which are shifted to the right,that, as a laser device ages, it is possible to compensate for reducedlaser power content by increasing the drive current supplied to thelaser. This aspect of semiconductor laser operation is relevant for anunderstanding of the different laser power levels used in the systemshown in FIG. 15 and is additionally relevant for an understanding ofsome of the monitoring and compensation functions that can be performedin conjunction with the use of the tracking and accuracy aspects of thepresent invention.

FIG. 17 illustrates yet another embodiment of the present invention. Inparticular, the embodiment shown in FIG. 17 employs dithering at twodifferent frequencies ω_(R) and ω_(T). More particularly, it is seenthat laser source 83 supplies a beam to beam splitter 104. As inprevious embodiments, one part of this beam is used for readinginformation within various levels of the recording medium and the otherportion of the beam, namely, that which is directed to reflective means85, is referred to as a tracking or reference beam and is directed toreference layer 34 within the recording medium, as illustrated in FIGS.5, 6, 7 and 8. The tracking beam is moved back and forth in anoscillatory manner by dithering modulator 74 which operates at afrequency ω_(T). Dithering modulator 74 preferably comprises apiezoelectric device or a voice-coil-activated vibration device. Theresulting reflected signal is supplied to beam splitter 104 returningupon reflection from the medium and is directed into semiconductor lasersource 83. It is particularly noted that, in this embodiment, lasersource 83 is operating as both a source and as a detector in the mannerdescribed above. The embodiment illustrated in FIG. 17 is employedprimarily for the writing or creation of information on rotating diskmedium 30. Information reading circuitry is not shown for the purposesof FIG. 17 which, as indicated, is essentially directed to writingoperations.

Of particular note is the fact that laser light source 83 also operatesas a detector for the returning reference beam and produces signalswhich have multiple components centered at different frequencies. One ofthese components is centered around the vicinity of frequency ω_(T) ;the other is centered around frequency ω_(R) which is introduced intothe system through the operation of focus dithering means 101 which istypically implemented as a lens moving towards and away from the mediumat a frequency ω_(R) or which is somehow varying its focal parameters atthe same frequency. These frequencies are chosen so as to be readilyseparatable from one another by ω_(T) filter means 102 and ω_(R) filtermeans 103. These filters are preferably standard band pass, high-pass,low-pass (or combinations thereof) filters as appropriate and may beeither active or passive circuits. In typical operation, ω_(T) ispreferably between about 5 and 50 KHz while ω_(R) is preferably betweenabout 0.5 and 5 KHz. Two band pass filters 102 and 103 may be employedor a high pass and low pass filter may also be employed as long as thefrequency ranges do not significantly overlap. In this way, two separateposition signals are provided to laser source position control unit 81.This unit positions source 83 in a radial direction as a result of thesignals provided from ω_(T) filter 102. Likewise, position controlelement 81 moves laser light source 83 towards or away from the mediumto control focus in a desired level by means of the signal that isprovided by ω_(R) filter 103. Thus, tight control is provided forpositioning laser light source 83 in a radial direction on the disk andalso for positioning the light source at an appropriate distance awayfrom the medium so that its focus remains steadily within a selectedlayer. Clearly, in the situation shown in FIG. 17, position control unit81 includes the tracking detection circuitry shown above in FIG. 13.Also of note in FIG. 17 is that beam splitter 104 is shown herein as aprism to indicate the use of such a device as an alternate to thestandard, simpler beam splitters shown elsewhere herein.

From the description provided above concerning the manner in whichrecordable media useable in the present invention may be manufactured,it is clear that there is nothing in the cited process which prohibitsrecordable surfaces from being present on both sides of a rotatablemedium. Accordingly, it is seen that it is possible to read and writeinformation from opposites sides of the same disk. An assembly foraccomplishing this objective is particularly illustrated in FIG. 18.This figure particularly illustrates the presence of layers 35a and 35bon a first side of the medium and layers 35c and 35d on the opposite(second or obverse) side of the medium. In such an embodiment, systems,such as are described above, are employed in duplicate or in parallelwith two lasers being disposed on opposite sides of the medium so thatit can have both of its sides read from or written to at the same timein an independent manner. Nonetheless, when, principly for reasons ofeconomy, it is desirable to employ only a single laser beam, such asystem is constructed using the mechanism shown in FIG. 18. Inparticular, beam splitter 85 divides the light from a laser source intotwo beams, one of which is directed to a proximal side of the rotatingmedium through shutter 86 and the other of which is directed to thedistal side of medium 30 through shutter 87. In general, when shutter 86is open, shutter 87 is closed and when shutter 87 is opened, shutter 86is closed. This provides isolation for the reading and writing ofinformation from and to the medium. Nonetheless, it is necessary toemploy a mechanism for redirecting light from one side of the disk,around its edge, to the other side. This mechanism is provided bymoveable frame 114 which supports mirrors 111, 112 and 113 as shown.(Additionally, in alternate embodiments, a fiber optic cable in amoveable mount is provided as a mechanism for redirecting laser lightfrom one side of medium 30 to the other side.)

Attention is next directed to FIG. 19. FIG. 19 illustrates the mechanismof FIG. 18 in the more complete context of an information writing systememployed in conjunction with other aspects of the present invention. Inparticular, it is seen that, when the beam is split by means of beamsplitters 85a and 85b, as shown in FIG. 19, it is necessary to employshutters 86 and 87 positioned as shown. The system shown in FIG. 19 isparticularly desirable for precise control of writing information ontomedium 30. In particular, it is seen that shutters 86 and 87 do notinterfere with the return of a tracking signal to photodetector 84 whichis provided to ω_(T) filter 102 as shown. Additionally, for reasons ofsimplicity, clarity and ease of understanding, function block 117essentially incorporates the two functions of position and intensitycontrol as is more specifically illustrated in FIG. 15 as functionblocks 81 and 91, respectively. Accordingly, FIG. 19 represents theincorporation of the mechanism shown in FIG. 18 in an apparatus usedprimarily for writing information onto storage medium 30.

In some embodiments of the present invention and, in particular, insituations similar to that shown in FIG. 21, it is sometimes desirableto produce two parallel beams of laser light. Apparatus 130 shown inFIG. 20 achieves this purpose by providing the effects of two prisms 131and 132 disposed with respect to one another as shown in FIG. 20. Thismechanism provides another means for splitting an input light signalinto two separate signals which are meant to travel on essentiallyparallel paths.

The embodiment of the present invention shown in FIG. 21 is particularlydirected to a system and method which permits simultaneous reading andwriting of information. For a proper understanding of this aspect of thepresent invention, it is important to appreciate that, while partiallyfunctional, the illustration shown in FIG. 21 is also in part athree-dimensional isometric view which should be read and interpretedwith that perspective in mind. Accordingly, it is seen that the systemin FIG. 21 includes laser light source 83 which directs a thin laserbeam to beam splitter 121 which is positioned and angled to direct partof the beam, namely, the tracking or reference beam, to reflective means123 which, in turn, directs the tracking beam to an edge of referencetrack 34. Motion of reflective means 123 produces dithering as describedabove. It is the reflection of this reference beam back to reflectivemeans 123 and through beam splitter 121 to photodetector 125 whichpermits the generation of an appropriate tracking signal. In short, thesignal S×I is produced by photodetector 125.

However, it is important to note that, in addition to the usual trackingfunction, the system shown in FIG. 21 performs a read-after-writeoperation. This is accomplished through the use of a second beamsplitter 122 which is angled and positioned so as to direct the beamfrom laser source 83 directly onto a spot within recording medium 30 tobe written into or onto. In FIG. 21, it is seen that such a beam extendsdirectly upward from beam splitter 122. At the same time, a relativelysmall portion of this beam is directed to reflective means 124 whichprovides a light beam signal for reading information which has just beenwritten to the disk or which may have been written to the disk at anearlier time. This information modulates the light reflected from thedisk and the reflected light is directed by reflective means 124 back toangled and positioned beam splitter 122 to photodetector 126 whichprovides, not the S×I signal, but instead, the information signal. As inprevious embodiments, reflective means 123 is dithered at an appropriatefrequency ω_(T) for tracking purposes.

Reading after writing provides significant advantages to the system. Inparticular, when it is known what bit pattern is being written to thedisk, it is immediately therefore possible to determine that, in fact,that bit pattern has been indeed actually written to the disk. Moreparticularly, in those situations employing error correction coding,error detection circuitry is used to analyze to the electrical signalfrom photodetector 126 to immediately determine that an error ininformation storage has occurred.

With particular reference to the present system, the fact that onethereafter knows that a particular position on a disk has beenincorrectly written makes it possible to create directories andsubdirectories of information which is stored on the disk itself (orelsewhere) which specifically indicates bad tracks and track "sector"positions. This is particularly true in the present invention since theaccuracy of position determination renders it possible to store thelocation of each and every bit on the disk so that, in effect, the diskbecomes completely addressable almost in the random access memory senseof the word although not quite as fast.

The embodiment of the present invention shown in FIG. 22 is similar tothe systems shown in FIGS. 10 and 15, discussed above, but which moreparticularly includes circuits for performance monitoring. Thismonitoring is possible because of the return reflection of the trackingbeam from tracking layer 34 through lens system 71 to mirror means 66 tobeam splitter 67 and finally to photodetector 69. The existence of thissignal provides a mechanism for monitoring the performance of laser 65.In particular, it is noted that the system shown in FIG. 22 isparticularly adapted for the writing of information to medium 30. Thisis particularly relevant since the question of the aging of laser source65 is more relevant to the writing of information than it is to thereading of information from a disk.

One would like to be able to measure the performance of laser source 65over a relatively long period of time to determine whether or not thelaser should be replaced (or the drive current adjusted. As discussedabove, with time, the power-current curve shown in FIG. 16 shifts to theright. If the same level of drive current is supplied to semiconductorlaser source 65, then the power output is diminished and,correspondingly, the voltage appearing as an output of photodetector 69decreases. The aging of laser source 65 can, thus, at least be partiallycompensated by an increase in the drive current to a more satisfactoryvalue. The signal S×I appearing as the output of photodetector 69provides an opportunity for measuring a signal which is indicative ofthis aging process. There are several different ways to determine,control and compensate for this phenomenon. For example, if the maximumvalue of the output signal from photodetector 69 drops below apredetermined threshold, then microprocessor 155, receiving such asignal from maximum value detector 152, is programmed to increase thelevel of current supplied by current intensity control unit 91. Similarcontrols are effected if the minimum value, as supplied by minimum valuedetector 154, falls below a predetermined threshold. Microprocessor 155is programmed to permit the selection of either one (or both) of thesetwo criteria as a mechanism for increasing (or even decreasing)semiconductor laser current.

However, a more realistic appraisal of the laser-aging process over aperiod of time is specifiable by a modulation index E which is definedas (V_(nax) -V_(min))/(V_(max) +V_(min)). If this value of E as thusdefined drops below a predetermined threshold value, then it can beconcluded that laser 65 has in fact aged to a point which meritsreplacement or which will merit replacement in a relatively short,predictable period of time. A typical plot of modulation index E as afunction of time which is typical for semiconductor lasers isillustrated in FIG. 26. This curve suggests that at a point in time whenthe value of E drops below a critical value, E_(crit), then the laserdevice should be replaced. In the event that laser source 65 comprises aplurality or array of semiconductor laser diode devices, then thepassage of E to a value below E_(crit) is also usable to provide asignal that a different laser source in the array should be used.

The circuits that effectuate this measurement control and determinationare shown in FIG. 22. In particular, maximum value detector 152 andminimum value detector 154 determine maximum and minimum values for thesignal S×I over a relatively short period of time ranging fromapproximately tens of microseconds to about hundreds of microseconds.These detectors provide a representation of this maximum value tomicroprocessor 155 which stores these values in (preferablynon-volatile) storage 158. Clearly, a non-volatile store 158 ispreferred since, over a relatively long period of time, say 100,000hours, the storage device in which the present invention is present maybe turned on and off so that, over that period of time, power is notpresent at all times for memory preservation purposes. Furthermore, thelong term storage of these values provide microprocessor 155 with theability to compute a history of aging and performance-related data foreach laser source. Microprocessor 155 is therefore also usable tocontrol the level of semiconductor laser drive current in unit 91 as ameans for, at least initially, compensating for decreased laserperformance.

The system shown in FIG. 22 has a number of advantages. A significantadvantage is the ability of the system to predict the imminent arrivalof the end of life of the laser device (or one of them in the case of anarray). This occurs as the value of E decreases along the "knee" of thecurve shown in FIG. 26. However, a much more important utilization ofthe signal S×I comes from the fact that a form of laser diode that, inthe so-called "VCSEL" device (Vertical Channel Semiconductor EmittingLaser 300' in FIG. 27B). These devices are to be contrasted withedge-emitting laser diodes, 300 in FIG. 27A. These two different kindsof laser devices (300 and 300') are illustrated in FIGS. 27A and 27B. Inboth of these figures, laser light leaving partially silvered mirror 306is focused by lens 355 to form a focused beam 320 of usable laser light.However, the most important thing of note is that in an edge-emittinglaser diode 300, as shown in FIG. 27A, it is possible, because somelight escapes from partially mirrored surface 304 at the rear of thedevice, to provide a separate monitoring photodetector such as detector350 shown in FIG. 27A. Since there is an escape of light from the rear,albeit small, it is nonetheless possible to monitor the performance ofthe laser over a period of time. However, in the construction ofvertical channel semiconductor emitting lasers as shown in FIG. 27B, itis seen that the fabrication of these devices on a common substrateprecludes access to the rear of such devices. In such devices, all lightis emitted from lasing channel 305 contained in semiconductor body 302and exits through partially transmissive mirror 306. Accordingly, forsuch devices, the utilization of the signal S×I, in the mannerillustrated in applicants' FIG. 22 and in the manner discussed abovewith reference to FIG. 22, becomes that much more important forperformance monitoring over a period of time.

The utilization of VCSELs offers challenges which the present inventionsolves. Accordingly, the present invention makes VCSELs that much moredesirable. In particular, because VCSELs tend to be more temperaturesensitive than edge-emitting diodes, it is possible, using the presentinvention, to more closely control the value of the current drive. Inparticular it is more readily possible now to control the current sothat it operates at a low threshold current level. For VCSELs, thiscurrent level is approximately 1 ma. in contrast to levels of 20-30 ma.which are required for edge-emitting devices. Additionally, lowthreshold VCSELs tend to be temperature sensitive which means that theamount of output power produced by these devices can vary. Accordingly,mechanisms which provide for constancy of this power in terms of amonitorable and measurable feedback loop can in fact produce stableconditions in otherwise potentially unstable devices. Accordingly, theperformance monitoring in terms of maximum and minimum output values isparticularly useful and desirable when the laser source being used is aVCSEL. Accordingly, the feedback mechanism of the present invention, inparticular microprocessor 155, is used to control the value of E. Thisis important not only for VCSELs but also for the presently more commonedge-emitting laser diode.

Another possible source of error and misalignment can exist in rotatablestorage media in which laser light sources are employed either forreading or for writing information. In particular, there can be anangular misalignment between the laser light source and the surface ofthe rotating medium. Three variations of alignment are shown in FIGS.23A, 23B and 23C. In particular, it is noted that FIG. 23B illustrates acase of ideal alignment with respect to the angle at which the laserlight impinges upon the rotating medium.

It is noted, however, that if the angle is off to the "left" as is shown(in exaggerated form) in FIG. 23A, an out of focus signal, such as thatshown in FIG. 24A, is produced. In a similar vein, if the angularmisalignment is to the "right," as shown in FIG. 23C, then the outputfrom laser source/detector device 83 is shown in FIG. 24C. ("Left" and"right" are used here as relative terms which refer to a view seen by afixed observer.) These signals are similar to those produced for edgetracking, as seen in FIGS. 9A and 9B above. As above, the same circuitsare employed for detecting these signal waveforms to identify theexistence and degree of angular misalignment. This misalignment istypically caused by wear in drive motor bearings, distortions in thelens systems or warping in the disk itself. Nonetheless, in an idealsituation, access of the rotating disk should be essentially parallelwith the optical access of the laser system which produces the impingingradiation for reading or writing. Accordingly, a system such as thatdiscussed above can be employed for determining this verticalmisalignment.

In order to achieve this objective, it is desirable to employ a lenssystem which provides, for tracking purposes, a variation in angularalignment as is suggested in FIGS. 23A-23C. In accordance with one ofthe embodiments of the present invention, this variability can beproduced by disposing a lens, such as lens 162 in FIG. 25, withinrotatable cylinder 161 so that it is canted at a slight angle withrespect to the cylindrical access which also serves as an access ofrotation for the entire cylinder/lens system. It is noted that, whileFIG. 25 illustrates the use of a cylinder, any sufficiently rigid framemay be employed. The only requirements are that it should be capable ofbeing rotated and also capable of holding lens 162 in a canted position.Such a system is well suited for determining angular misalignment.

Monitoring of the signal S×I also provides other possible advantages ininformation storage systems as contemplated herein. In particular,systems employing laser light for reading and writing purposes cansuffer degradation over time in the intensity of output produced by thelaser. This is particularly true for semiconducting lasing devices.

In an edge-emitting laser device such as shown in FIG. 27A, it ispossible to employ photodetector 350 disposed behind rear, partiallyreflecting surface 304. This provides a mechanism for direct monitoringof light strength signals. However, in more recently developedvertical-cavity-surface-emitting lasers (VCSELs) such as are shown inFIG. 27B, it is not possible to employ photodetectors to monitor lightbeing emitted from a rear wall of the lasing device. In VCSELs, suchlight is blocked by underlying semiconductor body 308. Therefore, adifferent mechanism is needed for monitoring performance and agingcharacteristics associated with VCSELs employed as sources of laserlight.

Fortunately, it is possible to monitor a quantity referred to as themodulation index to determine proximity to end of life for semiconductorlaser devices. A plot of modulation index as a function of time is shownfor a typical semiconductor laser device in FIG. 26. When the modulationindex approaches a reduced value E_(crit), it is seen that the end ofthe useful life of the laser device is approaching. In general, thismodulation index E is defined as the ratio between two quantities. Thedenominator of the ratio is the sum of the maximum and minimum values ofthe voltage produced by the photodetector which measures the S×I signal.The numerator of the modulation index determining the ratio is thedifference between the maximum value of this voltage and the minimumvalue of this voltage. Accordingly, if the output of the photodetectoris measured in terms of a voltage, the modulation index is E=(V_(max)-V_(min))/(V_(max) +V_(min)).

In accordance with one embodiment of the present invention, themodulation index E is monitored over a period of time. When the value ofE falls below a critical value, E_(crit), such as that illustrated inFIG. 26, a warning is provided, or alternatively or in conjunctiontherewith, power in increased to the laser device to correct for theaging characteristics. This is therefore seen to prolong the life of thesystem. It is also seen to obviate the need for service in which it isnecessary to replace one or more of the lasing devices employed.

In systems in accord with the present invention, lasing devices aretypically the edge-emitting semiconductor laser devices as shown in FIG.27A or the vertical cavity devices shown in FIG. 27B. For example, inedge-emitting semiconductor laser device 300, semiconductor materialbody 302 is provided with doped layer of lasing material 305 whichdefines the cavity in which the lasing action occurs. Partiallyreflecting surfaces 304 and 306 are provided at each end. As discussedabove, monitoring photodetector 350 may be employed at the rear of thelaser source behind rear, partially silvered reflector 304. However, therelative reflectivity of surfaces 304 and 306 are provided in a mannerin which laser radiation 320 emanates through surface 306 and is focusedby lens 355. Electrical contacts to the lasers are made in accordancewith well-known industry practices.

In other embodiments of the present invention, it is desirable to employvertical cavity semiconductor laser devices such as those illustrated inFIG. 27B. These devices have the advantage that they are more readilymanufacturable and, in particular, are manufacturable so as to bedisposed in either a linear or rectangular array lasing elements. Theprinciple difference between the structures shown in FIG. 27A and FIG.27B is that the latter lasing devices 300' are oriented vertically andare fabricated on opaque substrate 308. As described above, it is thepresence of this substrate which precludes monitoring of the lasingaction from behind over time by a detector, such as detector 350 shownin FIG. 27A.

The system of the present invention also has other advantages and usagesgrowing out of the great accuracy with which the information trackposition is determinable. In particular, the system provides a mechanismfor monitoring not only the performance of the laser devices themselves,but is also capable of monitoring the performance of the drive motoremployed to rotate recordable medium 30. In addition, the system of thepresent invention also enables the creation of recordable media whichhave recorded thereon indications of the precise speed at which theinformation was written to the disk.

Some of the latter aspects of the invention are illustrated in FIG. 28which is similar to FIG. 15 which has already been described above.However, certain additional components are provided in this moreadvanced system for the purposes of achieving objectives described moreimmediately above. In particular, during a read or write controloperation, clock 151 is started or stopped as a mechanism for providingan indication of writing time or reading time T to microprocessor 155.Additionally, a very accurate value for the position R of the track in aradial direction is also provided to microprocessor 155. As data is readout from the disk, bit counter 152 provides microprocessor 155 with acount N of the number of bits which have been read. This counter can bereset as necessary under control of microprocessor 155. Microprocessor155 is also coupled to non-volatile store 153 for the purpose of storingvalues of R, T and N as these values are sampled over a period of time.Non-volatile store 153 may also include values of L which represents thephysical distance between bits written to the disk.

A process of carrying out these determinations and computations isillustrated in FIG. 29. In particular, the process begins with themeasurement of the value of R, the radial position for a selectedinformation track (Step 200). At the same time, a value for the linearvelocity V is also determined from values N, L and T since it is knownthat the linear velocity is in fact proportional to the ratio NL/T (Step210). Next, a value is determined for the angular velocity ω which isequal to the linear velocity V divided by radius R (Step 220). Thesevalues of R, V and X are stored in non-volatile storage or even writtento disk 30 together with a time indicator. Next, stored values of ω arecompared against one or more characteristic motor life curves (Step240). If the comparison is unsatisfactory, then an end-of-life motordrive warning is provided and/or a time-to-failure projection isindicated. Additionally, it is noted that, even without comparison tostored motor life characteristics, stored values for ω may be analyzedby themselves to determine if the variability in ω is greater thandesirable for information storage systems having certain capacity and/ordata transfer rates. End-of-life warning signals may be provided, forexample, if the value of ω has dropped by a certain percentage of itsoriginal value or if it has dropped below a value which is acceptablefor a drive motor having an age which is indicated by the time index(clock value) which is stored with the sample values of ω.

Clearly, if the information with respect to motor drive life is writtento disk 30 instead of to a non-volatile storage unit, then, it isnecessary that this same disk be employed in the storage system whenmotor life characteristics are being determined. Accordingly, suchsystems may be provided with special disks used to maintain service andmaintenance information. Accordingly, the present invention includesspecial diagnostic disks which are usable with one or more drive unitsto perform maintenance monitoring and diagnostic functioning.

The determination of linear velocity as described above in Step 210 ismore particularly illustrated in FIG. 30. In particular, the process ofdetermining linear velocity V begins by reading information bits frommedium 30 (Step 211). Once the reading of information bits is begun, aclock (such as clock 151) is started (Step 212). During readingoperations, strings of information bits are produced (Step 213). Thesebits are counted, as for example by bit counter 152, to produce aninteger N indicating the number of bits in the string or stream (Step214). At a convenient point in time, the reading of the information bitsis stopped (Step 215), at which time the clock is also stopped togenerate an elapsed time (Step 216) that has been required to read theseN bits. From the values of N and T and a previously known and storedvalue of L, the linear velocity V is determined as at least beingproportional to NL/T (Step 217).

As indicated above with respect to Step 240, stored values of ω=V/Rversus time may be compared with one or more previously known and/ordetermined motor life characteristics. Such a characteristic isillustrated in FIG. 31 which shows the typical variation in angularvelocity as a function of time. As with the laser light output asdiscussed above, once the value of angular velocity drops below thecritical value, ω_(crit), there is provided an excellent indication thatmotor failure is imminent or that the motor is sufficiently aged so asto preclude its further utilization. In situations where one is writinginformation to disk which is permanently stored thereon, it is importantto be able to determine that the writing system, namely, the motor driveand the laser drive mechanisms, are operating in a fashion which doesnot write useless and/or unretrievable information to the disk. Thiscould render the disk medium incapable of being read from or written toby other drive mechanisms into which it is inserted.

The mechanism of the present invention is also employed to detect wobblein the drive and spindle mechanism. In particular, at any given radialposition, a wobbling motion will produce a variation in focus in the Zdirection as a function of the angular position of the disk. Clearly,the determination of wobble is more sensitive at the outer tracks on themedium. Nonetheless, at virtually any radial position, variability ofthe focus of the laser light in the Z direction provides an indicationthat wobbling motion is occurring. With particular respect to thepresent invention, the ability to determine accurately values of R makesit very easy to determine whether or not the degree of focus variationprovides an indication that the wobble is beyond acceptable limits. Forexample, a given indication of wobble magnitude should be considered inlight of the radial position at which it is measured since it is knownthat focus variation is more sensitive at outer radial positions (highervalues of R). Thus, the system of the present invention is alsoapplicable to wobble monitoring.

Next is considered the application of the present invention in systemsand methods for position determination in the angular (θ) directiontogether with methods for determining performance of the system in termsof potential wobble. As above, all of these operations take fulladvantage of characteristics and improved accuracy associated with thesignal S×I.

Wobble is determined by measuring the height Z of the focal plane at asmall value of the radius R and by measuring it again at a larger valueof R. The difference between these two measured focal plane heights is ameasure of wobble. If it exceeds a predetermined value, there is thus anindication that there is a drive motor or drive mechanism problem forwhich an appropriate signal is generated.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art.Accordingly, it is intended by the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

The invention claimed is:
 1. An apparatus for writing information on andfor reading information from a recording medium which has at least oneset of substantially parallel light reflective tracks thereon and whichis susceptible to being modified by impinging laser light, saidapparatus comprising:a source of laser light; means for controlling theposition of said laser light source; a first beam splitter for dividinglaser light from said source into a first beam for transmitting saidinformation and into a second beam for tracking; means for varying thepoint of impingement of said second beam back and forth across saidtrack edges in an oscillatory manner at a predetermined frequency; afirst photodetector for receiving light from said second beam which hasbeen reflected from said medium and for producing therefrom anelectrical signal indicative of second beam position with respect tosaid tracks; tracking detector for receiving said electrical signal fromsaid first photodetector and for providing signals to said laser lightsource position control means; a second beam splitter for receiving saidfirst beam and for directing a portion of said first beam to writableportions of said medium between said tracks and for directing, to asecond photodetector, portions of said second beam which have beenreflected from information- bearing portions of said medium; means forcontrolling power output from said source of laser light in response todata input to cause said first beam to write information on said mediumin response to said input; a switch for enabling said means forcontrolling power output from said source of laser light and forpreventing signals from said second photodetector from reaching a datadetector; and said data detector being capable of interpretingelectrical signals from said second photodetector during readoperations.